Progressive cavity pumps, also called Mono pumps, PCP pumps, or Moineau pumps, are a type of displacement pumps which are commercially available in a number of designs for different applications. In particular, these pumps are popular for pumping high-viscosity media. Typically, such pumps include a usually metallic helical rotor which is termed, in what follows, the inner rotor, with Z number of parallel threads which are called thread starts in what follows, Z being any positive integer. The rotor typically runs within a cylinder-shaped stator with a core of an elastic material, a cavity extending axially through it being formed with (Z+1) internal thread starts. The pitch ratio between the stator and rotor should then be (Z+1)/Z, the pitch being defined as the length between adjacent thread crests from the same thread start.
When the geometric design of the threads of the rotor and stator is in accordance with mathematical principles written down by the mathematician Rene Joseph Louis Moineau in, for example, U.S. Pat. No. 1,892,217, the rotor and stator together will form a number of fundamentally discrete cavities by there being, in any section perpendicular to the centre axis of the rotor screw, at least one point of full or approximately full contact between the inner rotor and the stator. The central axis of the rotor will be forced by the stator to have an eccentric position relative to the central axis of the stator. For the rotor to rotate about its own axis within the stator, also the eccentric position of the axis of the rotor will have to rotate about the centre axis of the stator at the same time but in the opposite direction and at a constant centre distance. Therefore, in pumps of this kind there is normally arranged an intermediate shaft with 2 universal joints between the rotor of the pump and the motor driving the pump.
The pumping effect is achieved by said rotational movements bringing the fundamentally discrete cavities between the inner surfaces of the stator and the outer surfaces of the rotor to move from the inlet side of the pump towards the outlet side of the pump during the conveyance of liquid, gas, granulates etc. Characteristically enough, internationally these pumps have therefore often been termed “PCPs” which stands for, in the English language, “Progressive Cavity Pumps”. This is established terminology also in the Norwegian oil industry, for example.
The volumetric efficiency of the pump is determined mainly by the extent to which these fundamentally discrete cavities have been formed in such a way that they actually seal against each other by the relevant rotational speed, pumping medium and differential pressure, or whether there is a certain back-flow because the inner walls of the stator yield elastically or because the stator and rotor are fabricated with a certain clearance between them. To increase the volumetric efficiency, progressive cavity pumps with elastic stators are often constructed with under-dimensioning in the cavity, so that there will be an elastic squeeze fit.
Not very well known and hardly used industrially to any wide extent—yet described already in said U.S. Pat. No. 1,892,217 —are designs of progressive cavity pumps in which a part, like the one termed stator above, is brought to rotate about its own axis in the same direction as the internal rotor. In this case the part with (Z+1) internal thread starts may more correctly be termed an outer rotor. By a fixed speed ratio between the outer rotor and the inner rotor, both the inner rotor and the outer rotor may be mounted in fixed rotary bearings, provided the rotary bearings for the inner rotor have the correct shaft distance or eccentricity measured relative to the central axis of the bearings of the outer rotor.
A limitation to the gaining of ground of such early-described solutions has probably been that an outer rotor needs to be equipped with dynamic seals and rotary bearings, which is avoided completely when a stator is used. On the other hand, an intermediate shaft and universal joints may, in principle, be avoided when the stator is replaced with an outer rotor.
In U.S. Pat. No. 5,407,337 is disclosed a Moineau pump (here called a “helical gear fluid machine”), in which an outer rotor is fixedly supported in a pump casing, an external motor has a fixed axis extending through the external wall of the pump casing parallel with the axis of the outer rotor in a fixed eccentric position relative to it, and the shaft of the motor drives, through a flexible coupling, the inner rotor which has, beyond said coupling, no other support than the walls of the helical cavity of the outer rotor, the material is assumed to be an elastomer. In this case the rotation of the outer rotor is driven exclusively by movements and forces at the contact surfaces of the inner cavity against the inner rotor. A drawback of this solution is that if there is considerable clearance at or elastic deflection of the contact surface, the inner rotor or the outer rotor will be moved more or less away from its ideal relative position. Further, by increasing load, the driving contact surface between the inner and outer rotors will be moved constantly nearer to the motor and force the inner rotor more and more out of parallelism relative to the axis of the outer rotor, so that over the length of the outer rotor, the inner rotor will contact the outer rotor on diametrically opposite sides with consequent friction loss, wear on rotors and motor coupling and also possible signs of wedging. Vibrations, erratic running and reduced efficiency may also be expected.
In U.S. Pat. No. 5,017,087 as well as WO99/22141 inventor John Leisman Sneddon has shown designs of Moineau pumps, in which the outer rotor of the pump is enclosed by and fixedly connected to the rotor of an electromotor whose stator windings are fixedly connected to the pump casing. In these designs the outer and inner rotors of the pump are both fixedly supported radially at both ends in the same pump casing, so that the outer and inner rotors of the pump function together as a mechanical gear, driving the inner rotor at the correct speed relative to the outer rotor which, in turn, is driven by said electromotor. In this case as well, signs of wedging between the inner and outer rotors may arise, in particular if solid hard particles seek to wedge between the inner and outer rotors where these have their driving contact surfaces. Besides, a disadvantage of an inner rotor fixedly supported at both ends is that if the pumping medium is of a kind which must be separated from contact with the bearings, independent dynamic seals will be needed at both ends for both the inner rotor and the outer rotor, as these do not have a common rotary axis.
In U.S. Pat. No. 4,482,305 is shown a pump, flow gauge or similar according to the PCP principle with inner and outer rotors. Here is used a wheel gear outside the pump rotors which ensures a stably correct relative rotational speed between the inner and outer rotors, independently of internal contact surfaces between them. This ensures smoother running, in particular by great pressure differences and/or spacious clearances—which may be necessary to achieve a gradual pressure increase when compressible media are pumped. However, it is assumed here as well that there are dynamic seals and radial bearings at both ends of the inner rotor. The dynamic seal for the outer rotor is also complicated by the diameter of the sealing surface having to be large enough to allow an internal passage for both the pumping medium and the bearing shaft on the extension of the active helical part of the inner rotor.
The invention has for its object to remedy or reduce at least one of the drawbacks of the prior art.
The object is achieved through features which are specified in the description below and in the claims that follow.